1. Field of the Invention
The present invention relates to a control technique for a communication system, and more particularly, to a control technique for a wireless communication system.
2. Description of the Related Art
Continuing growth in wireless telecommunications usage is placing increasing stress on the capacity of cellular systems which provide wireless telecom service. The limited frequency spectrum available for cellular communications demands that cellular systems have increased network capacity and adaptability to various communications traffic situations. Although the introduction of digital cellular systems has increased potential system capacity, these increases alone may be insufficient to satisfy added demand for capacity. Other measures to increase system capacity, such as decreasing the size of cells in metropolitan areas, may be necessary to meet growing demand.
It is known in the prior art that localized microcells and picocells may be established within overlying macrocells to handle areas with relatively dense concentrations of mobile users, sometimes referred to as “hot spots”. FIG. 1 illustrates an example of a macrocell 10 served by a basestation 20 including two hot spots 30. Typically, microcells may be established for thoroughfares such as crossroads or streets, and a series of microcells may provide coverage for major traffic arteries such as highways. Microcells may also be assigned to large buildings, airports, and shopping malls. Picocells are similar to microcells, but normally cover an even smaller area, such as an office corridor or a floor of a high-rise building. The term “microcell” is used in this application to denote microcells, picocells, and any other “inner” layer of another cellular structure. The term “macrocell” is used in this application to denote the outermost layer of a cellular structure. An “umbrella cell” can be a macrocell or a microcell as long as there is a cell underlying the umbrella cell. Microcells allow additional communication channels to be located in the vicinity of actual need, thereby increasing cell capacity while maintaining low levels of maintenance.
Macrocell umbrella sites typically cover radii in excess of 1 kilometer and serve rapidly moving users, for example people in automobiles. Each microcell site is usually served by a separate low power, small radio base station, with its own antenna that is located within the microcell and which primarily handles slow moving users such as pedestrians. Each microcell site is connected to a macrocell site through a transmission medium, such as digital radio transmission or optical fibers. Microcells suffer from a series of problems, including an increased sensitivity to traffic variations, interference between microcells, and difficulty in anticipating traffic intensities.
As the capacity of cellular systems has increased, cell sizes have decreased, in some networks to as small as 0.5 km radius, such that controlling co-channel interface has become a major problem. The use of microcells is a way of increasing capacity still further. In a microcellular layout, base station antennas are placed below the building height in urban areas, and low power is used such that the propagation characteristics between base station and mobile are dominated by the street layout. Interference from adjacent cells may be blocked by buildings.
Microcellular techniques allow significantly higher traffic densities to be achieved, and also enable smaller, lower power mobiles to be used. The use of microcells requires improved handover, i.e., handoff, techniques, which allow for fast and reliable handoff, for example when turning a street corner. One way of easing handover problems is to employ an “umbrella cell” arrangement using conventional cells overlaying the microcells such that handover can be made into the umbrella cell where no suitable adjacent microcell can be identified. This also avoids the need to plan a contiguous microcell coverage in an urban area.
The current solution for dealing with the hot spot problem illustrated in FIG. 1 is to create microcells within the macro-cells, i.e., to introduce hierarchy in the cell. As illustrated on FIG. 2, two microcells 40 are formed at the two hot spots 30. The macrocell 10 acts as the umbrella cell. Such a solution requires installing basestations 20′ in each hot spot coverage area, which is costly. Moreover, this solution assumes that the hot spots 30 do not move with time.